Despite the use of adjuvant treatments for breast cancer, it is common for patients to succumb to metastatic disease. Metastatic disease arises principally from disseminated tumor cells (DTC) that have been shed from primary tumors and localize in lymph nodes and organs such as bone marrow. DTC can progress to micrometastases through a complex series of steps. About 20% of 5-year breast cancer survivors will ultimately relapse in years 5-10 post-treatment. These recurrences often arise in bone. The presence of DTC is a significant risk factor in reducing the life-expectancy of patients. Therefore, a key goal for radionuclide therapies of cancer is to develop strategies to sterilize DTC in bone. Radium-223 dichloride, a drug that emits potent alpha particle radiation, was recently approved by the FDA for treatment of advanced metastatic prostate cancer in bone. Radium-223 binds to bone surfaces and only cancer cells that lie within the ~100 m range of alpha particles are irradiated directly. Yet, research has shown that irradiated cells can generate signals that cause biological changes in neighboring cells that have been hit by little or no radiation. The nature of these changes can depend on the radiation dose. Alpha particles produce the most potent of these bystander effects and cell killing can be a consequence. Therefore, bystander effects may play an important role in killing or otherwise altering the behavior of DTC that are beyond the range of the alpha particles. We hypothesize that chronic alpha-particle irradiation of bone and surrounding tissue by radium-223 causes dose- dependent bystander effects that result in reduction of proliferation, migration and invasion rates of bystander DTC in bone marrow, and increased killing of bystander DTC. In addition, we hypothesize that these bystander effects will alter the radio sensitivity of the DTC to subsequent radiotherapy treatments. We will test these hypotheses with experiments conducted with human breast cancer cells in innovative organ culture and in vivo animal models. The proposed studies are carried out in four interrelated, yet independent aims. The information gained from Aims 1 and 3 will enhance our understanding of the efficacy of radium-223 therapy and whether bystander effects play a role. It may translate into strategies that enhance its effectiveness in killing DTC. It will reveal whether non-targeted effects can be exploited with radium-223 in a clinical setting. Recognizing that the future of radium-223 therapies may ultimately evolve from palliative to front-line therapies, the experiments in Aim 2 will guide how radium-223 may impact the efficacy of subsequent radiotherapies. Importantly, Aim 4 develops novel small scale dosimetry that will make it possible to identify bystander cell populations and correlate their responses with absorbed dose rate and absorbed dose to the neighboring tissues. In summary, our efforts seek to gain knowledge in support of an expanded therapeutic role for radium- 223 and alpha particle emitting radionuclides in general. Our proposed studies are in direct response to recent statements that the potential role of bystander effects in radionuclide therapies is underexplored.